Global surface pressure data from 1976 to 1997 from over
7500 stationsand COADS marine reports have been analyzed using harmonic
and zonalharmonic methods. We find that the diurnal pressure oscillation
S1 is comparableto the semidiurnal pressure oscillation S2 in magnitude
over much of the globeexcept for the low-latitude open oceans, where S2 is
about twice as strongas S1. Over many land areas, such as the western United
States, the TibetanPlateau, and eastern Africa, S1 is even stronger than
S2. This is in contrast to the conventional notion that S2 predominates
over much of the globe. The highestamplitudes (~1.3 mb) of S1 are found over northern South
America and easternAfrica close to the equator. S1 is also strong (~1.1
mb) over high terrain such theRockies and the Tibetan Plateau. The largest amplitudes
of S2 (~1.0-1.3 mb) arein the tropics over South America, the eastern and western
Pacific, and the IndianOcean. S1 peaks around 0600-0800 LST in low latitudes
and around 1000-1200LST over most of midlatitudes, while S2 peaks around
1000 and 2200 LST overlow-and mid-latitudes. S1 is much stronger over the land
than over the ocean andits amplitude distribution is strongly influenced by
land masses, while the land-seadifferences of S2 are small. The spatial variations of
S1 correlate significantly withspatial variations in the diurnal temperature range at
the surface, suggesting thatsensible heating from the ground is a major forcing for
S1. Although S2 is muchmore homogeneous zonally than S1, there are considerable
zonal variations inthe amplitude of S2 which can not be explained by zonal
variations in ozone andwater vapor. Other forcings such as those through clouds'
reflection and absorptionof solar radiation and latent heating in convective precipitation
are needed to explainthe observed regional and zonal variations in S2. The
migrating tides S11 and S22 predominate over other
zonal wave components. However, the non-migrating tidesare substantially stronger than previously reported.
The amplitudes of both themigrating and non-migrating tides decrease rapidly poleward
with a slower pace atmiddle and high latitudes.